Gas turbine engines include one or more compressors for pressurizing a working medium fluid, typically ambient air, that flows through an axially extending compressor flowpath. Under some operating conditions, it is desirable to temporarily moderate the pressure at the discharge end of the compressor to prevent or recover from compressor stall or other aerodynamic instabilities. Pressure moderation is usually effected by opening a valve that diverts a portion of the pressurized fluid from the discharge end of the compressor flowpath into a lower pressure region.
An exemplary compressor valve is described in U.S. Pat. No. 4,827,713, which is assigned to the assignee of the present application, and whose contents are incorporated herein by reference. The disclosed valve includes a stationary orifice ring having a series of circumferentially distributed passages joining the compressor flowpath to a surrounding annulus, and a valve ring for regulating fluid flow through the passages. A bellcrank mechanism for operating the valve ring is mounted on a mechanically grounded bellcrank support bracket by a bellcrank pivot. The bellcrank pivot, whose details are not illustrated in the reference, comprises a pivot pin that connects a clevis portion of the bellcrank to the bracket. The pivot pin has a cylindrical shank extending through apertures in the clevis and through a corresponding bore in the bracket. A pair of equally sized bushings circumscribes the pin and lines the bore. Input and output arms of the bellcrank are connected respectively to an actuator (not illustrated in the reference) and to the valve ring. The bellcrank arms are axially offset from the axial midplane of the clevis due to space constraints not evident in the reference.
In operation, the actuator rotates the bellcrank mechanism about the bellcrank pivot so that the bellcrank, in turn, drives the valve ring in a spiral motion to cover or uncover the passages. The pin rotates in unison with the bellcrank so that, ideally, relative motion occurs at the interface between the interior surface of each bushing and the exterior, cylindrical surface of the pivot pin. The pivot also reacts the transverse forces exerted by the actuator, and by the mechanical resistance of the valve ring, to transversely support the bellcrank in the bracket. Because the bellcrank arms are offset from the axial midpoint of the pivot, the transverse forces are apportioned unequally between the bushings so that a larger transverse load is exerted on the bushing more closely aligned with the bellcrank arms and a smaller transverse load is exerted on the other bushing. The larger load exerted on the aligned bushing results in a proportionately larger frictional force resisting rotation of the pin and the bellcrank. The larger frictional force can cause locally accelerated wear of the pin and can eventually cause the pin to seize in the bushing or to fracture, effectively disabling the valve or at least degrading its response to the actuator. The above described problem can be addressed by frequently inspecting the support pin for signs of excessive wear and incipient seizure. However the bellcrank mechanism is not readily accessible in a fully assembled engine, and so pin inspection is both time consuming and expensive and therefore is a highly unsatisfactory solution to the problem.
In view of the above described shortcomings an increased reliability, high durability bellcrank mechanism is sought.